Allow setting the config dir to use as enviroment variable for phpunit

This allows for setting the config to use for running the tests either by setting the env variable in the command line or in the phpunit.xml:

```xml
<php>
	<env name="CONFIG_DIR" value="tests/config/mysql"/>
</php>
```

This makes it easy to have multiple test "profiles" to test for example different db backends without having to manually switch the config file around or having multiple installations with different config files

cc @karlitschek @DeepDiver1975 @PVince81